Various designs are known of bi-axial positioning devices using electromagnetic drive systems.
The closest possibility consists in an arrangement using two mono-axial positioning systems where the second system is mounted on the movable component of the first system. This arrangement takes its cue from the customary way of designing compound tables, as applied for instance in machine tool manufacture. Applied to electromagnetic drive systems, this principle f.i. is known in several variations for the German Letters Patent No. 24 36 900. In all these designs it is of disadvantage that the lower system must always drive the entire upper drive system, i.e. that there are relatively large masses that must be moved in the direction of a coordinate.
Also known are arrangements unifying the planes of movement of both drive systems, i.e. avoiding additional mechanical parallel guides. These arrangements considerably reduce the moving masses, since the reactive component of both drive systems is standing still relative to the movement of both axes, and additional guiding elements are eliminated.
The arrangement described in the German Letters Patent No. 22 65 246 is known, where the active component of the positioning device carries the drive magnets for both coordinate directions. The reactive component, the base plate, has parallel slots, so that the surface, like a waffle iron, is covered by cube-shaped islands which are generating motive forces in conjunction with the pole systems. The system is supported on air. The disadvantage of this arrangement lies in the fact that due to the crosswise arranged slots, the forces that can be achieved will reach only one half of the value that could be achieved if the slots were arranged, like a line grid, in the direction of one coordinate. It is furthermore of disadvantage that two complete and identical pole systems must be used for every direction of movement in order to avoid torsional vibrations.
This reduction of force is avoided in the arrangement described in the U.S. Pat. No. 3,851,196. The plane of movement has therein separate zones for the drive systems of both coordinate axes so that instead of the waffle grid, a line grid can be used which will allow greater reactive forces. It is of disadvantage here, that for reasons of symmetry the base plate must be given at least three separate grid zones, which necessitates a base plate of very large dimensions when compared to the effective zone of movement of one axis. This design of the movable head necessitates two pole systems in one direction, in order to avoid torsional vibrations. In the other direction, the resulting force will act upon the center of gravity of the head, and only one pole system is required. It is the purpose of the invention, on one hand to utilize possibly small dimensions of the base plate for a given working range of the positioning device, and on the other also to avoid the reduction of force occurring by the use of the waffle grid.